The present invention relates to a process for hydraulically fracturing a geological formation along a predetermined direction.
Fracturing of a geological formation is sometimes employed to establish communication between two wells at the level of the geological formation. This communication is, for example, established to achieve underground gasification of a coal bed whose permeability is insufficient to ensure the gas flow rate required between the two wells to sustain backward burning. Fracturing of geological formations is also employed in the field of enhanced hydrocarbon recovery processes wherein a pressurized fluid is injected from injected wells into the geological formation to cause hydrocarbon transfer toward production wells. As a matter of fact, it may be desirable in this case to improve fluid injection or hydrocarbon recovery by fracturing the geological formation along a direction which is preferably perpendicular to the direction of flow of the fluid.
This fracturation, which can establish a communication on the one hand between the injection wells and/or, on the other hand, the production wells, thus improves the scavenging of the geological formation by the injected fluid.
It is already known to fracture a geological formation traversed by a well by injecting a hydraulic fluid at a sufficient pressure at the level of the geological formation. The direction of the so-created fracture mainly depends on the field or tensor of the pre-existing stresses in the geological formation. In the most favourable cases this direction is known more or less with accuracy. The wells to be connected by fracturing are then positioned along this direction.
In spite of this, experience shows that the so-achieved fracturation does not always correspond to the desired one and, for example, cannot interconnect two wells whose locations are remote from each other.
U.S. Pat. No. 3,270,816 describes a method for fracturing a soluble geological formation so as to interconnect the two wells. According to this method a notch is created in the wall of each well so that the fracture develops from these notches when pressure is established in the wells. These notches are postitioned so that the cracks developed from each well are at an angle with the plane containing the axes of the two wells, so that two secant cracks are created. Experience has shown that this method is not suitable for insoluble geologic formations.
Other methods have been described to create networks of cracks perpendicular to each other so as to establish communication between several wells.
One of these methods is described in U.S. Pat. No. 3,682,246 which teaches conducting two successive pressurizing steps in one and the same well for fracturing the formation along two perpendicular directions. Experience has shown that this double fracturing of one and the same well cannot be effected in practice.
Another method, described in U.S. Pat. No. 3,709,295, makes use of three wells aligned along the natural direction of fracturing. The two lateral wells are hydraulically fractured, then the central well is hydraulically fractured while the lateral wells are kept under pressure. This is supposed to induce a crack at right angles to the preceding fractures. Experience and calculations have shown that injection of hydrualic fluid into the fractures induced from the lateral wells leads to a modification in the stress field in the vicinity of the central well by rendering this field isotropic. As a consequence, the direction of fracturing at the location of the central well cannot be ascertained.
According to a third method, described in U.S. Pat. No. 4,005,750, it is possible to create a network of cracks intersecting one another so as to interconnect a plurality of wells.
To this end, a first well is fractured along its natural direction of fracturing, then, while keeping the pressure at the same level in the first well so as to maintain the fractures open, a second well is fractured so as to induce therefrom second cracks which intersect the first fracture. The operating steps are then repeated starting from the second well and there is thus obtained in a step by step manner a network of mutually perpendicular cracks.
Thus, none of these prior techniques provides a fracturing along a single predetermined direction which may differ from the natural direction of fracturation.